As you are new to plumbing, unless you feel really confident, i'd get a plumber in. For an expert this job can be done in a couple of hours with the heating side either still full or drained halfway.
Also don't think you'll find a towel rad with the same dimensions so you will prob have to alter the pipework.
But if you want a go. Switch off your combi and find out where the drain off for your heating supply is. Its usually located at the lowest point in the system. As you are new i'd drain down the whole system to prevent any unexpected floods!! Open some rad bleed screws to help the system run out
Once the heating side is empty and dry - remove your old bathroom radiator and the old rad brackets.
Mark out where you want your new towel rad to go. Make sure everything is level for the towel rad brackets. Drill and screw the towel rad holders to the wall. Add the tails and valves and remember to use plenty of PTFE. Hang the rad and mark up for your pipes. I use chrome pipe to match- dont try and solder or use pushfit to this!!-compression only. Refit your heating flow and return but dont put your floorboards back yet.
Tighten up all rad bleed screws and refill the heating system slowly. Stop and check for any leaks. If youve used compression and find any just 'nip' them up. Bleed your rads and continue filling via your combi filling loop until youre up to 1.5 bar. Run your heating system and again walk around checking for any leaks. Then you can put back any floorboeards youve removed.
See -- easy peasy
Add inhibitor via the bleed valve at the top and tighten up.